The present invention relates to adaptive optics control and in particular, to a state space wavefront reconstructor for use in an adaptive optics control system and a method of compensating for the distortion of an optical wavefront using such a state space wavefront reconstruction.
Optical wavefronts are subject to distortion when passed through certain mediums, such as the turbulent atmosphere. These distortions can degrade the quality of images of an object being observed through an image-forming device such as a camera or telescope. In poor conditions, such distortions and degradations can be especially severe and problematic. Some of the diffractive effects of such turbulence include image blurring caused by the turbulence aberrations, scintillation caused by propagation through strong turbulence, and speckle caused by coherent scattering from diffuse objects.
Various adaptive optic control systems, mechanisms and methods have been developed to try to correct for the blurring that results from such distorted wavefronts. Examples of various such systems are disclosed in Adaptive Optics for Astronomical Telescopes, by John W. Hardy, Oxford University Press, 1998 (pages 31–33, and 55–69), which is incorporated herein by reference. An adaptive optic control system typically includes a wavefront phase slope sensor for measuring the phase differences or phase slopes between points of a wavefront, a wavefront reconstructor for estimating the wavefront phase from the phase differences, a control system for reducing the effects of noise, and a wavefront corrector for correcting the wavefront based thereon.
The wavefront phase slope sensor is usually in the form of a Hartmann wavefront sensor as shown in FIG. 2, although other sensors, such as a lateral-shearing interferometer, may be used. Hartmann wavefront sensors use an array of lenslets or a mask pierced with an array of holes for dividing the distorted wavefront into an array of subapertures. Each of the beams of the subapertures is focused onto one or more detectors disposed behind the holes or lenslets. When the distorted wavefront passes through the holes or lenslets, it forms an array of spots on the detectors which are indicative of the wavefront slope or tilt, if any at each corresponding subaperture. Typically, the wavefront phase slope sensor includes an analog-to-digital converter and one or more processors to compute the wavefront phase slopes.
The wavefront corrector is usually in the form of a deformable mirror which comprises a thin reflective surface to the back of which a plurality of actuators are secured. The actuators expand or contract in length upon application of a voltage or a magnetic field in accordance with the electrical commands generated by the wavefront reconstructor, thereby pushing or pulling on the faceplate and causing the mirror to change its shape to make the appropriate corrections to a distorted waveform passing through it. The actuators are typically arranged in a square or hexagonal array defining a plurality of zones, and are capable of displacing the faceplate locally within each zone by a few micrometers up or down.
Accurate reconstruction of the wavefront for control of the deformable mirror is the key sub-system in an adaptive optics system for high-end telescopes such as those used for imaging, laser-beam projection and astronomy. The phase estimation performed by the wavefront reconstructor, however, is limited by the quality of the data available. While all sensor measurements are subject to noise, there can be additional difficulties in accurately measuring the wavefront phase. Partially-filled lenslets (such as those near the aperture or near interior obscurations) cause stretching of the spot size, which can change the responsivity of a subaperture. Strong aberrations in the beam at a distance from the telescope can cause a variation in the intensity of the telescope known as scintillation. Such aberrations can be caused by high-altitude atmospheric turbulence or in optical systems looking horizontally, such as airborne laser systems. Reflection of coherent light from a distributed object can also vary the intensity of the telescope. This is known as speckle.
Scintillation and speckle also often lead to points around which the phase appears to be discontinuous when in fact it may not be. These points are known as branch points. The effect of a branch point on a wavefront is to introduce a non-zero curl; that is, the sum of the phase slopes around the branch point is a non-zero multiple of 2π. When scintillation and/or speckle are present, the quality of the data used by the wavefront reconstructor is degraded. In particular, scintillation and speckle corrupt the measurements made by the wavefront phase slope sensor by changing the spot shapes and sizes on the focal-plane pixels of the wavefront corrector, which causes a non-linear response. At lenslets where the intensity goes to zero, it is possible to have multiple spots at the focal plane array within a Hartmann detector subaperture, resulting in a badly corrupted measurement.
In benign conditions, the reconstructor may be a least-squares estimator, as described by Hardy or in William H. Press, et al, “Numerical Recipes in C: The Art of Scientific Computing,” by William H. Press, Cambridge University, 1992, also incorporated herein by reference. However, the least-squares approach ignores branch points, and so the reconstructor's performance is degraded by them. Existing methods that attempt to identify and correct for branch points are usually challenged by the quality of the data. This causes the identification of branch-points to be ambiguous, and often leads to branch point false alarms. In such cases, a correction is made for a non-existent branch point, which degrades the phase estimate calculated by the wavefront reconstructor.
Various reconstructors and adaptive control systems have been developed to try to correct or compensate for the problems associated with scintillation and speckle. These approaches to wavefront phase estimation/reconstruction typically include temporal or spatial filters to suppress noise. However, the ambiguities caused by corrupted data rather than by noise cannot be eliminated simply by increasing the signal-to-noise ratio or by filtering the resulting estimates. In addition, existing reconstructors only support estimations of present states. Scintillation and speckle errors, however, change over time as the wavefront moves across the aperture.
Accordingly, there is a need for an improved wavefront reconstructor that more accurately estimates the phase of a wavefront in an optical system, especially in strong turbulence conditions.